FUNCTIONS OF THE NUTRIENTS 159 



ment demands an abundant supply of protein food. It 

 is also true of mature animals that sufficient protein is 

 not only necessary to health and vigor, but it is essen- 

 tial to production that is satisfactory in quantity and 

 quality. 



230. Protein as a source of fats. The functions of 

 protein are not restricted, however, to the use already 

 described, for it is utilized in more ways than any other 

 class of nutrients. It was held at one time that outside 

 the vegetable fats it is the sole source of animal fats, and 

 this view was, not so very long ago, to some extent 

 accepted. Indisputable proof to the contrary is now in 

 our possession. Certainly we must be convinced that 

 nitrogen compounds of the food are, with some species, 

 not the most important source of animal fat, for various 

 investigators, such as Lawes and Gilbert, Soxhlet, and 

 others, have shown upon the basis of searching experi- 

 ments that sometimes over four-fifths of the fat stored 

 by pigs must have had its origin outside the food protein 

 and fat. Jordan showed that milk-fat may be largely 

 synthesized from carbohydrates. Besides all this, the 

 common experience of feeders that foods highly non- 

 nitrogenous are often the most efficient for fattening 

 purposes is good evidence that fat formation is not 

 greatly dependent upon the protein supply. Neverthe- 

 less, the possibility of producing animal fat from protein 

 now appears to be demonstrated. 



231. Protein as a source of energy. Protein can 

 unquestionably serve as fuel, or, in other words, as a 

 source of m energy. The amount so used depends much 

 upon the animal fed and the character of the ration. In 

 the case of a dog eating an exclusive meat diet or of a 

 mature fattening animal which receives a ration liberally 



